libftdi: rename option to have proper prefix

The option name BR2_PACKAGE_LIBTFDI_CPP obviously had a typo: it
should have been named BR2_PACKAGE_LIBFTDI_CPP, and add the necessary
Config.in.legacy handling.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>